Control of excess chromium in malleable irons



United States Patent Office 3,419,439 Patented Dec. 31, 1968 3,419,439 CONTROL OF EXCESS CHROMIUM IN MALLEABLE IRONS Price B. Burgess, Albion, Mich., assignor to Malleable Research and Development Foundation, Dayton, Ohio, a corporation of Ohio No Drawing. Filed Feb. 14, 1966, Ser. No. 527,098 5 "Claims. c1. 148-3) ABSTRACT OF THE DISCLOSURE Ferritic malleable iron is produced from a white iron having 0.05 to 0.25% chromium, 0.003 to 0.05% aluminum, 0.0015 to 0.03% boron, actual manganese less 1.7 times the sulfur content of not more than 0.2%, 1.4 to 2% silicon, 2.0 to 3.0% carbon, in the range of 0.175% copper, not more than 0.18% phosphorus, and not more than 0.1% molybdenum. By compensating for the inhibiting amount of chromium with aluminum and boron in the amounts specified, with silicon and manganese in the range indicated, it is possible to use conventional annealing cycles rather than varying the cycle for each heat in order to compensate for the interfering amount of chromium. By the process herein described, ferritic malleable having nodule counts in the range of 75 to 250 nodules per square millimeter is obtained. Various cycles and procedures are described as well as physical data as to the resulting ferritic malleable.

This invention relates to a method for the production of malleable irons from white iron, and more particularly to the production of malleable irons having an excess of chromium over that normally present in white iron.

Malleable iron includes two principal types standard or ferritic malleable which includes ferrite in which is interspersed nodules of free carbon, and pearlitic malleable in which some of the carbon is present in combined form. The production of malleable iron is a direct process in which scrap, foundry returns and the like are the raw materials. Melting is carried out in a cupola, air or induction furnace or in combinations, for example, a duplexing system. Metallurgical inspection is closely controlled during melting to provide the proper chemical composition of the white iron. Following the melting operation, the white iron castings are poured and heat treated for malleabilization, the heat treatment or annealing being variable depending upon whether ferritic or pearlitic malleable iron is the final product.

As cast White iron of malleable composition Will solidify with the carbon which is present in the material being in the form of cementite or iron carbide, and when at room temperature, will consist of rather large carbides and pearlite, that is, alternate layers of ferrite and cementite. The malleabilization procedure converts the combined carbon into elemental carbon, that is, graphite or temper carbon and ferrite. In first-stage malleabilization or graphitization, white iron castings are heated through the eutectoid range to transform the pearlite into austenite in which carbon from the cementite dififuses into the iron to form a solid solution of carbon and gamma iron.

The first-stage graphitization includes several processes which are carried out simultaneously including solution of the cementite at its interface with austenite, dissolution or disassociation of cementite into iron and carbon, migration of carbon through the austenite or diffusion of matrix atoms away from the nuclei from which the temper carbon grows, and precipitation of graphite. After first-stage graphitization, the structure of the casting consists of graphite, also referred to as temper carbon nodules, which are distributed through the austenite matrix, the latter being a solid solution of gamma iron saturated with an amount of carbon which is dependent upon the articular temperature of the first-stage malleabilization procedure. Usually the first-stage malleabilization is carried out at a temperature of between 1600 and 1800 F.

The second stage consists of reducing the temperature of the iron into the eutectoid range wherein the iron exists in three phases, austenite-ferrite-cementite, or austeniteferrite-graphite. The first phase is considered metastable while the second is considered a stable phase. At a temperature slightly below the eutectoid range, any pearlite in the iron will graphitize. By slow passage through the eutectoid range, the iron is fully graphitized or malleabilized and no further structural change takes place at the lower temperatures. The product of full malleabilization is ferritic malleable iron which is substantially free of pearlite structure.

The objective in the formation of pearlitic malleable is to treat the product of the first-stage graphitization in such a manner that the eutectoidal carbides and low-temperature transformation products are purposely retained. Various procedures have been used commercially in the formation of pearlitic malleable iron including first-stage graphitization, air quench, and temper; or followed by the additional steps of reheating into the austenitic range, oil quench and temper, or ferritic malleabilization (ferrite and free carbon) followed by reheating into the austenitic range, oil quench and temper. Recently, several malleable iron foundries have used a continuous procedure which includes first-stage graphitization, cooling, oil quench and tempering. The matrix may vary, for example, it may be lamellar pearlite, spheroidite, martensite, tempered martensite, fine spheroidite, coarse lamellar pearlite or bainite.

Chromium has been considered to have an inhibiting effect on malleabilization of white iron, that is, it is a strong carbide stabilizer preventing conversion of the combined carbon to elemental carbon. The presence of chromium in an amount in excess of 0.05% generally requires a change in the annealing cycle, that is, the temperature treatment sequence used to bring about malleabilization of white iron. Since annealing is generally carried out in commercial practice on a continuous basis, the presence of an inhibiting amount of chromium required changes in temperature settings in the annealing ovens from one heat to the next depending upon whether or not chromium was present in the white iron within tolerable limits, generally not above about 0.04%.

Since scrap materials are used as a portion of the starting material in the production of white iron, it has been the practice to use scrap with a low residual chromium, for example, below 0.03%. There is however some contamination in each batch of the scrap material because of the presence of alloyed materials, or stainless steels in which chromium may be present in an amount as much as 25% even though the batch of scrap material is classified as unalloyed. The normal chromium maximum for unalloyed scrap is considered to be 0.15% and the Institute of Scrap Iron and Steels specifies 0.20% chromium maximum for scrap free of alloys.

Thus, the manufacturers of malleable irons are faced with the alternative of using scrap having a relatively high amount of residual chromium and adjusting the annealing cycle, or paying a premium price for scrap having a low residual content. Because of the continuous nature of the annealing process and the control of white iron composition generally maintained in the foundry, the practice heretofore has been to pay the premium price for scrap containing low residual chromium.

In accordance with the present invention a process and product have been provided in which the percentage of chromium in white iron may be as much as 0.25%, substantially higher than the inhibiting amount, and wherein the composition of the mixture is adjusted prior to freezing thereof to compensate for the interfering amount of chromium. The term freezing as used in this application means solidification of the molten material to form white iron. The presence of chromium in an inhibitory amount is compensated for by the addition of aluminum and boron while maintaining the actual manganese content less than 1.7 the sulfur content at a relatively low level. In this way, the white iron of malleable composition is provided which may be annealed using the conventional cycles currently being used in malleable foundries. A representative analysis of malleable iron in accordance with the present invention includes between 0.05% and 0.25% chromium, and preferably between 0.05% and 0.15%, between 0.003% and 0.05% of aluminum, and preferably between .003% and 0.01%, between 0.0015% and 0.03% boron, and preferably between .0015 and 006%, actual manganese less 1.7 times sulfur of not more than 0.2%, and preferably between 0.10% and 0.15%, between 2.0% and 3.0% carbon, and preferably between 2.2 and 2.8%, between 1.0% and 2.0% silicon, and preferably between 1.4% to 2.0%, between 0.020% and 0.175% copper, not more than 0.18% phosphorous, not more than 0.1% molybdenum and balance iron.

Accordingly, it is a primary object of the present invention to provide a method of forming malleable irons from white iron in which chomium is present in an interfering amount, for example, between 0.05% to 0.25%, and wherein the white iron has added to it aluminum and boron so that the annealing cycle used is that used when chromium is present in a non-interfering amount, or less than 0.05%.

Another object of the present invention is the provision of a method for forming malleable irons from white chromium tending to prevent conversion'of combined carbon in the white iron to elemental carbon, and wherein the heating cycle used during malleabilization is substantially the same as that used with white iron having chromium present in an amount less than an interfering amount.

Other objects and advantages of the invention will be apparent from the following description and the appended claims.

In accordance with the present invention, the composition of the white iron and the annealing cycles thereof may be varied within standard annealing cycles, and excess chromium can be compensated for as will be described herein below. Typical annealing cycles included 14.65 hours in a continuous production furnace having an inert atmosphere, the sequence being as follows: heat to 1750 F. for 3.30 hours, hold to 1750 F. for 5.50 hours, cooled to 1350 F. for 1.10 hours and cooled to 1250 F. for 4.75 hours. Other cycles included a 17.3 hour cycle in a continuous production furnace running on an inert nitrogen gas atmosphere, and the same cycle in the laboratory furnace using an atmosphere of carbon monoxide and carbon dioxide generated by coke. The sequence for the 17.3 hour cycle was heated to 1750 F. in 2.7 hours, hold at 1750 F. for 4.3 hours, cool to 1650 F. in two hours, cool to 1360 F. for 1.3 hours, and cool to 1260 F. in seven hours. Batch cycles of 54 and 85 hours were as follows:

Heat to 1,700 F Hold to 1,700 F. to l,735 F Cooled to 1,300 F r The 85 hour cycle was used to study the possible pre- TABLE I Wet Analysis, Percent 1 Speetrographio Analyses, Percent l Anneal Completion, Heat Percent Noduln No. Count 2 Cr C Si Mn S Cr P Ni Mo Al B Cu V Atmos. 1st 2nd 1-... 0. 027 2. 49 1. 57 0. 53 0. 115 0. 020 0. 009 0. 038 0. 014 0.003 0. 0012 0. 041 07 78 2 0. 042 2. 53 1. 27 0. 51 0. 154 0. 046 0.017 0. 041 0. 018 0. 002 0. 0017 0. 065 75 86 3 0. 047 2. 52 1. 54 0.40 0. 150 0. 047 0.023 0. 045 0. 013 0. 004 0. 0019 0. 156 100 58 4..- 0. 098 2. 49 1. 57 0. 46 0. 117 0. 067 0. 011 0. 038 0. 014 0. 003 0. 0022 0. 046 1 1); 5 0. 130 2. 54 1. 53 0. 38 0. 147 0. 124 0. 022 0. 044 0. 013 0.006 0. 0019 0. 159 6 0. 140 2. 52 1. 20 0. 52 0. 141 0. 137 0. 018 0. 041 0.020 0. 004 0. 0020 0. 065 15 18% 1 Titanium and tin were below 0.01% for all samples and remainder was iron. 2 Nodule Count-N odules per square millimeter.

iron, and a malleable iron casting wherein the white iron includes an interfering amount of chromium, not greater than about 0.25%, in which aluminum and boron are added in an amount of at least 0.0015% and 0.002% respectively, and wherein the manganese content less 1.7 times the sulfur content is not greater than 0.2%.

Another object of the present invention is the formation of ferrite or pearlitic malleable from white iron having an interfering amount of chromium, wherein the white iron is annealed by essentially the same cycle used if the chromium were present in a on-interfering amount.

A further object of the present invention is the provision of a method of bringing about at least 95% malleabilization of a white iron of malleable composition wherein the white iron includes interfer amounts of Other examples of the present invention included heats with a base iron including 2.5% carbon, 1.5% silicon, 0.15% chromium and 0.08% nickel. Two hundred pound heats were made and each heat was split into five taps and inoculated as follows: (1) no inoculation, (2) 0.04% calcium, silicide plus 0.04% bismuth, (3) 0.004% ferrosilicon plus 0.04% bismuth. (4) 1.01% bismuth and (5) 0.0015% boron plus 0.01% bismuth. Standard tensile bars were cast and annealed by the 14.65 hour cycle previously described. After annealing three-quarter inch diameter test bars were examined metagraphically and for chemical composition. The inoculation was by addition to ladles, and the chemical analysis of the various heats is shown in Table II.

TABLE II [Deliberate variations italicized] Chemical Analyses, Percent Prior to Ladle Addition After B-Bi C Si S Ni Mn Cu B Al Bi B Bi 2. 50 l. 53 0. 110 0. 0. 26 0. 018 0. 0029 0. 0025 0. 0010 0. 0044 0. 0078 2. 55 1. 49 0. 094 0. 06 0. 28 0. 167 0. 0029 0. 0005 0. 0009 0. 0043 0. 0064 2. 54 1. 52 0. 112 0. 05 0. 28 0. 151 0. 0029 0. 0025 0. 0009 0. 0040 0. 0010 Table III shows the results of annealing for each of the five ladle additions of the heats in Table II.

TABLE III.PART IIRESULTS OF ANNEAL OF SECTIONS Nodule Count/sq. mm. Second Stage Completion, Percent Heat No High Level CaSi-Bi FeSi-Bi Bi B-Bi High Level CaSi-Bi FeSi-Bi Bi B-Bi Elements Elements 7 B, Al 126 123 125 120 130 B, A 100 100 100 99,5 100 Cu, B.-. 123 129 117 133 Cu, B s5 95 95 9o 99 9 Cu, B, AL-.. 143 130 125 112 115 Cu, B, AL... 99.5 99.5 99.5 99.5 100 In accordance with the present invention it has been These data show that with low manganese (030-035 determined that boron increases second stage annealing Examples 10-14) heats substantially complete annealing and aluminum is helpful in bringing about substantially complete annealing while aluminum and boron together behave significantly better than either separately. For example, compare heats 7 and 9 with 8 in each of Tables 11 and III supra.

In other examples of the present invention, 100 pound heats of the base iron used to pour the heat of Tables II and .III were made and cast into standard tensile test bars and two inch round mottle bars with and without inoculation with 0.0015% boron and 0.01% bismuth. The tensile test bars were annealed in the same continuous production equipment and on the 14.65 hour cycle previously described. Some two inch test bars were annealed with a 54 and 85 hour cycle on a batch basis. The data is presented in Tables IV, V and VI.

TABLE IV.CHEMICAL ANALYSES OF HEATS [Deliberate variations italicized] Chemical Analyses, Percent (Prior to Ladle Additions) Heat N 0.

Cr C Si S Mn B Al 1 Not analyzed; value shown is average of other heats made in a similar vay.

is obtained when boron exceeds 0.002% and aluminum 0.035%. In the case of high manganese heats, additional boron is required, about 0.005% while the minimum aluminum remains the same as for the low manganese heats. Also, in the case of the 54 and 85 hour cycles, the slower bring up time of 38 hours versus 19 hours has tended to result in higher nodule counts presumably because of the longer time in the prebaking temperature range, e.g. above 800 F. and less than the eutectoid range. This was true especially of the low manganese heats as shown in Table VII in which heat 15 is compared with heats 10 and 12.

TABLE VIL-EFFECT OF MANGANESE AND BORON ADDI TION ON PREBAKE" RESPONSE IN 2 ROUNDS-BATCH AN NEALED Change in N odule Count38 vs. 19 hrs. to Heat Excess 1,700 F.

No. Mn,*

Percent No Inoculant B+Bi Added *E M t 11 t" ll TABLE V.ANNEALING RESULTS- ROUNDSIn 14.05 mess anganese 15 M mess ER. CYCLE N0 Ladle Addition B+Bi Added Heat No. Nodule Second N odule Second Count Stage Count Stage Completion Completion 55 130 95 152 99.5 193 100 217 100 109 95 132 99 12% gg i2? 88 Table VIII shows mechanical properties of ferritic test 138 98 183 99 bars made from the heats as indicated in the table. In the 192 100 206 100 case of several of the heats, the multiple figures are taps 238 100 248 100 including the ladle 1noculants as lndlcated. TABLE VI.ANNEALING RESULTS-2' ROUNDS N0 Ladle Additions Boron+Bi Added as in Amount Above lgleat 14.65 Hr. Cycle 54 Hour Cycle 85 Hour Cycle 14.65 Hr. Cycle 54 Hour Cycle 85 Hour Cycle N.C.l Second N.C.l Second N.C./ Second N.C.l Second N.C./ Second N.C.l Second sq. mm. Stage sq. mm. Stage sq. mm. Stage sq. mm. Stage sq. mm. Stage sq. mm. Stage Excessive carbides-incomplete first stage (less than 09% complete I Reheat treated ferritic-oil TABLE VIIL-MECHANICAL PROPERTIES OF FERRITIC TEST BARS [99% or better annealed in section] Heat Nodules/ Tensile, Yield, Elong.

No. Ladle Add. sq. mm. p.s.i. p.s.i. BHN

Percent 7 None 126 48, 200 28, 200 11. 121 CaSi+B1 123 49, 000 30, 400 11. 5 126 FeSi+Bi 125 48,100 30, 000 11. 0 121 Bi. 120 47, 000 30,200 8. 0 121 B-i-Bl... 130 49, 000 28, 900 10. 5 121 9 None... 143 51, 200 31, 800 11. 5 121 CaSi+B 130 50,000 29,200 12. 5 131 FeSi+Bi 125 50, 300 30, 500 13. 0 126 Bi 112 51, 500 31, 300 13. 5 126 50, 500 30, 400 11. 5 126 TABLE IX.MECHANCIP%1-% PROPERTIES OF PEARLITIO BARS quenched from 1 600 F. and tempered 1% hours at 1,200 F. or 1,100 F.

Heat Ladle N odules, Tensile, Yield, Elong., BHN

No. Add. sq. mm. p.s.i. p.s.i. percent White iron of malleable composition including up to 0.15% of chromium may be successfully annealed with commercial heat treating cycles provided there is a balance of aluminum, boron, manganese and sulfur as previously described. Boron acts as a nuclei former while aluminum assists i graphitization. In accordance with the present invention it has also been noted that aluminum provides a high nodule count in the center of the sections and a relatively low count on the outside, while boron tends to cause a higher nodule count near the surface. Both boron and aluminum together appear to act as nodule count promoters.

The amount of excess manganese, that is, in excess of over 1.7 times the sulfur content is preferably maintained in the range of 0.10% and 0.15% in accordance with the present invention since this provides best annealing, particularly with white iron of high chromium content.

The significance of the present invention is the ability to use commercial standard annealing cycles with an excessive amount of chromium in the White iron, and compensating for the excessive amount of chromium by a balanced relationship of aluminum, boron, manganese and sulfur.

While the processes and products herein described constitute preferred embodiments of the invention, it is to be understood that the invention is not limited to these precise processes and products, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.

What is claimed is:

1. The method of forming ferritic malleable iron comprising the steps of forming an iron mixture of malleable composition wherein the percentage of chromium is between 0.05 and 0.25% thus tending to inhibit malleabilization, adjusting the composition of said mixture prior to freezing thereof to include between 0.0030% and 0.05% of aluminum and between 0.0015% and 0.03% boron, maintaining the actual manganese content less 1.7 times the sulfur content not more than 0.2%, maintaining silicon in the range of 1.0% to 2%, forming white iron castings from said mixture wherein substantially all of said carbon is combined as iron carbide and wherein said iron consists of iron marbides and pearlite, malleableizing said white iron to effect at least graphitization of said carbides and 95% transformation of said pearlite into ferrite thereby providing a structure of temper carbon nodules distributed through a matrix of ferrite, said castings having a nodule count of between 75 and 250 nodules of temer carbon per square millimeter, and maintaining said castings in an inert atmosphere during malleabilization to prevent decarbonization thereof.

2. The method as set forth in claim 1 wherein said mixture includes 2% to 3.0% carbon, not more than 0.18% phosphorus, not more than 0.10% molybdenum, and between 0.020% and 0.175% copper.

3. The method as set forth in claim 1 wherein chromium is present between 0.05% and 0.15%, aluminum is present between 0.003% and 0.01%, boron is present between 0.0015% and 0.006%, and wherein the actual manganese content less 1.7 times the sulfur content is between 0.10% and 0.15%.

4. The method as set forth in claim 1 wherein the total amount of manganese is between 0.30% and 0.35%, boron being present between 0.0015 and 0.010%, and aluminum being present between 0.0015 and 0.06%.

5. The method as set forth in claim 1 wherein said malleableizing operation includes prebaking said castings at a temperature above about 700 F. and below the eutcctoid range of said white iron.

Physical and Engineering Properties of Cast Iron, Britrsh Cast Iron Research Assoc., Birmingham, England, 1960, relied on, pages 227 and 228.

Metals Handbook, 1948 Ed., pub. by ASM, relied on, pages 520523.

CHARLES N. LOVELL, Primary Examiner.

U.S. C1. X.R. 

